1. Field of the Invention
The present invention relates to a charging device for an image formation apparatus such as a duplicator, printer and facsimile telegraph, and more particularly, to a charging device having a simple structure which can uniformly charge a photosensitive medium, and a method of manufacturing the same.
2. Description of the Related Art
In general, a charging device for a photosensitive medium receives a predetermined current and generates a surface potential on the photosensitive medium using a current according to a corona method, a contactable roller method or a contactable board method.
In the corona method, a corona discharger is used as a charging unit uniformly charging a surface of the photosensitive medium. The corona discharger efficiently uniformly charges the surface of the photosensitive medium to a predetermined potential. However, the corona discharger requires a high voltage power, and generates ozone during a discharging operation. The ozone contaminates an environment, and the photosensitive medium drum and a charging member deteriorate.
In order to solve the foregoing problem, the contactable roller method employs a charging roller instead of the corona discharger. Referring to FIG. 1, a contactable roller type charging device includes a charging roller 20 driven in contact with the photosensitive medium 30. The charging roller 20 has a shaft 21. When a power supply unit 10 supplies a voltage to the shaft 21 of the charging roller 20, the charging roller 20 charges a surface of a dielectric substance. In the contactable roller method, the charging roller 20 is disposed to contact the photosensitive medium 30 to generate the surface potential on the photosensitive medium 30. That is, when the voltage of about −1 to −2 kV is applied to the shaft 21 of the charging roller 20, the charging roller 20 having a resistance of about ‘1×104Ω˜9×107Ω’ generates an electric discharge on a contact portion of the photosensitive medium 30 and the charging roller 20. The voltage is transmitted from the power supply unit 10.
When the photosensitive medium 30 receives the predetermined current, the surface potential is generated thereon according to a property of a surface dielectric layer of the photosensitive medium 30. The surface potential generated on the photosensitive medium 30 is in proportion to the voltage applied to the charging roller 20. When the shaft 21 of the charging roller 20 receives a direct current power, the surface potential of the photosensitive medium 30 is generated from an electric discharge start voltage, and increased in proportion to the voltage to a predetermined voltage level.
In the contactable board method, as illustrated in FIGS. 2 and 3, a conductive elastic member 40 is installed to contact the photosensitive medium 30 to generate the surface potential on the photosensitive medium 30. In order to maintain a predetermined contact nip, the conductive elastic member 40 is positioned to contact the photosensitive medium 30 with a predetermined contact linear pressure. In addition, the conductive elastic member 40 has a transformation (deflection) amount of ‘δ’ with the photosensitive medium 30 due to the contact linear pressure. When the power supply unit 10 transmits power to a high voltage unit terminal 11, the conductive elastic member 40 having a resistance of about ‘1×104Ω˜9×107Ω’ generates an electric discharge on the contact portion of the photosensitive medium 30 and the conductive elastic member 40 according to a resistance property.
When the predetermined current is transmitted to the photosensitive medium 30 due to the electric discharge, the surface potential is generated according to the property of the surface dielectric layer of the photosensitive medium 30. The surface potential generated on the photosensitive medium 30 is in proportion to the voltage applied to the conductive elastic member 40. A length and thickness of the conductive elastic member 40 are set up by a relational expression of Cantilever load and deflection, so that the conductive elastic member 40 and the photosensitive medium 30 can maintain the contact linear pressure constant.
The contactable roller method has been widely used to solve the problems of the corona method having a low energy efficiency, an ozone generation, and an irregular charging. That is, the contactable roller method lowers the voltage for charging the photosensitive medium and limits and reduces the ozone generation during the charging operation. Moreover, the contactable roller method prevents dust particles from being electrostatically deposited on a corona wire, and does not require the high voltage power.
However, the contactable roller method has disadvantages in that charging distribution is not uniform and a charging potential is very sensitive to the environment. As compared with the corona method using the corona discharger, the contactable roller method is not preferable in uniformity of the charging distribution. When the charging roller is left at a low temperature, an electric resistance of the charging roller is increased to reduce the charging potential of the photosensitive medium more than at a normal temperature and humidity by about 200V. Therefore, a reverse phenomenon causing ink blots on print matters may be generated.
In addition, the contactable roller method must perform complicated processes, such as extrusion and polishing process, in manufacturing the charging roller and uses special conductive materials to increase raw material expenses and process expenses. When the charging roller is maintained at a high temperature for an extended period of time, a low molecular weight material of the charging roller is migrated to the contact portion of the charging roller and the photosensitive medium, and thus, a horizontal band phenomenon is generated near the contact portion of the photosensitive medium. In order to minimize the migration of the low molecular weight material, the charging roller must be formed of a special resin, or an outer layer of the charging roller must be coated or tubed, which results in high prime cost. In the polishing process, the surface of the charging roller may be caved, and spot defects may be generated due to a pinhole of the surface of the charging roller. Moreover, the polishing, processing and coating processes of the charging roller must be precisely performed to prevent foreign materials from being introduced into the charging roller. Accordingly, the prime cost is increased, and mass production is hardly achieved.
As compared with the contactable roller method, the contactable board method is highly advantageous in cost. However, the contactable board method has difficulty in maintaining elasticity for the contact linear pressure between the conductive elastic member and the photosensitive medium and it is impossible to keep the contact nip between the conductive elastic member and the photosensitive medium. It is therefore difficult to uniformly maintain the surface potential of the photosensitive medium. In addition, the contact nip is hardly maintained in contacting and charging the surface of the photosensitive medium, especially a board using a cantilever method. Accordingly, the contactable board method is not stabilized in motion and rotation and needs improvements of the contact nip.